Hi:

My favorite are the "Jet Airliners" Tutorial articles by Jane Whittaker in one of the two prominent sim magazines (the one by Key Publishing). I highly recommend those extremely well-written and readable articles.

The articles specifically related to VNAV start at issue 49 for about 4-to-5 subsequent issues with titles such as: VNAV - The Vertical Navigation Tool - keep to the path, Mastering VNAV - managing your profile, Vertical Navigation and the FMC - VNAV'ing with experts,  and Climb and descent with autopilot - what goes up must come down etc. The series is ended w/ a Putting theory into practice tutorial flight where SPD interventions and FLAP controls during a (sharp turn) VNAV active climb are explained.

Hope you can get hold of those back-issues. You can, of course, also find other related references/resources of more technical nature elsewhere.

Have fun.